Discovery of BKT correlations in the quantum kagome compound Cs$_2$Cu$_3$SnF$_{12}$

TL;DR

This study reveals that the kagome compound Cs$_2$Cu$_3$SnF$_{12}$ exhibits Berezinskii-Kosterlitz-Thouless (BKT) correlations and a gapless ground state, with evidence of short-range chiral order, making it a unique quantum kagome system.

Contribution

First experimental evidence of BKT correlations and chiral order in Cs$_2$Cu$_3$SnF$_{12}$ using NQR measurements, highlighting its novel quantum magnetic properties.

Findings

Presence of BKT-type correlations above T_N

Gapless ground state confirmed by relaxation rate measurements

Short-range chiral order observed over a broad temperature range

Abstract

We investigate the microscopic properties of the kagome compound Cs$_2$Cu$_3$SnF$_{12}$ using $^{63,65}$Cu nuclear quadrupolar resonance (NQR). Analysis of the local hyperfine fields below the N\'eel temperature $T_N = 20$ K indicates a spin structure consistent with $P2_1/n$ symmetry of negative vector chirality. Measurements of the spin-lattice relaxation rate $T_1 ^{-1}$ reveal signatures of a gapless ground state and two-dimensional Berezinskii-Kosterlitz-Thouless (BKT)-type correlations above $T_N$, extending over a broad temperature range of approximately 130 K in zero magnetic field. Within the same temperature range, the observed increase in the NQR linewidth is consistent with short-range chiral order recently identified by neutron scattering. Our results establish Cs$_2$Cu$_3$SnF$_{12}$ as a unique quantum kagome system exhibiting BKT behavior.